Chemical vapor deposited (CVD) SiO2 films and their binary and ternary silicates find wide use in VLSI processing. These materials find use as insulators between polysilicon and metal layers, between metal layers in multilevel metal systems, as diffusion sources, as diffusion and implantation masks, as capping layers to prevent outdiffusion, and as final passivation layers.
The manner in which a thin film covers or conforms to the underlying features on a substrate is an important characteristic in semiconductor processing. Conformal coverage refers to coverage in which equal film thickness exists over all substrate topography regardless of its slope, i.e. vertical and horizontal substrate surfaces are coated with equal film thickness.
One manner of effecting the deposition of SiO2 on a substrate is through pyrolysis of an organic silicon precursor in a CVD reactor to form SiO2. A typical organic silicon precursor is tetraethoxysilane or TEOS which is represented by the chemical formula Si(OC2H5)4. A typical reactor used to effect the pyrolysis of organic silicon precursors is a low pressure CVD reactor or LPCVD reactor. LPCVD reactors include both hot wall and cold wall reactors. In hot wall reactors, wafers can be heated utilizing radiant heat supplied from resistance-heated coils. In cold wall reactors, wafers can be heated utilizing infrared lamps or rf induction.
LPCVD reactors are typically operated at pressures of around 0.25–2.0 Torr and temperatures of around 550° C. to 800° C., although such parameters may vary depending on a number of different conditions including the particular types of reactants used. The stoichiometry of decomposition of TEOS within an LPCVD reactor may be simplistically written as:Si(OC2H5)4→SiO2+4C2H4+H2O2
Typically, however, intermediates are formed in the above reaction which include di-ethoxysilane (Si(OC2H5)3OH) and tri-ethoxysilane (Si(OC2H5)2(OH)2). Further, other reaction by-products are formed.
One problem facing the semiconductor wafer processor is achieving adequate and conformal step coverage of deposited SiO2 into very deep and narrow contact openings or other so-called high aspect ratio topographies. One such substrate surface topography is depicted in FIGS. 1 and 2 and designated generally by reference numeral 10. Topography 10 is defined by a deep trench 12 into which an SiO2 layer 14 has been deposited as by CVD of a suitable organic silicon precursor such as TEOS.
FIG. 1 illustrates a situation in which adequate conformality has been achieved as evidenced by the uniformity or substantial uniformity in thickness of layer 14 over the substrate surface, and particularly within trench 12. FIG. 2 illustrates a situation in which inadequate conformality has resulted in non-uniformity in the thickness of layer 14, particularly at and near the bottom of trench 12. Such is an undesirable condition.
One source of inadequate conformality of SiO2 on a substrate surface is premature formation of undesirable intermediates which react to form SiO2 at higher topographical elevations on a substrate surface. Consequently, such intermediates never reach the bottom of a particular substrate feature, such as trench 12 of FIG. 2, so that lesser degrees of SiO2 are formed thereon.
One method to improve step coverage has been to increase pressures in the CVD reactor. By doing so, the partial pressure of the organic silicon precursor, such as TEOS, is increased, while the partial pressure of the intermediates is not. The increase in organic silicon precursor partial pressure results in improved step coverage because the precursor has a more favorable sticking coefficient as compared with the intermediates.
Another attempt to increase step coverage has been to introduce ethylene (C2H4) into the reactor with the precursor to inhibit the premature formation of intermediates. Unfortunately, great success has not been achieved due to significant degradation of deposition rates stemming from competitive absorption relative to the substrate surface as between the ethylene and the precursor.
This invention grew out of the need to provide improved step coverage of LPCVD SiO2 over high aspect ratio substrate topography.